For economy and ecological reasons, numerous efforts have been made heretofore to recover, from spent mixed acid (i.e. HF/HNO.sub.3) solutions, valuable chemicals which can be used, for pickling purposes, and to cope with serious ecologically undesirable waste sewage problems. Inherently, recycling of hydrofluoric acid and nitric acid would provide real economy if recovery thereof and recycling could be achieved economically.
Unfortunately, processes proposed and/or adopted in recent years are expensive, cumbersome, involved, or all of these.
Typically, one process which has been recommended would use, e.g., sodium fluoride to precipitate the double salt Na.sub.2 FeF.sub.5, separation of the latter and treating it with caustic soda in order to separate ferric hydroxide and recover recyclable sodium fluoride. The process is very highly expensive because of the presence of other metal fluorides and the requirement that a partial stream of any recycled solution be treated with caustic soda.
In another proposed process a strongly basic ion exchange resin is contacted by the spent pickling solution and the ion exchange resin washed with water. As with a chromatographic column, the effluent exhibits two partly overlapping concentration peaks, the first of which is due to the predominating quantity of salts and the second to the presence of free acids. An optimal separation of fractions allegedly enables a recovery of about 60% of free nitric acid present. The acid/salt fraction must be treated with neutralizing agents. This method likewise does not really solve the problem because the recovery rate is unsatisfactory.
In a third suggested method, the nitric and hydrofluoric acid residues combined with metal ions are first released by the addition of sulfuric acid in a corresponding quantity, and free acids plus dissolved molybdenum are extracted with a solution of tributyl phosphate in kerosene. The acids are eluted with water from the organic solvent phase and the resulting aqueous solution is treated with activated carbon, after which it is returned to the cycle as regenerated solution. Before the organic phase can again take up acids, molybdenum fluoride must be removed therefrom by treatment with a sodium hydroxide solution. The resulting basic solution, together with an extraction residue which contains a major part of the heavy metals, is first neutralized with lime and then with a sodium hydroxide solution. It is apparent from this short description that detailed technological problems are involved, as well as considerable expenditures for equipment, control mechanisms, and chemicals.
As is well known, spent pickling solutions contain various metal ions. In addition to ferric ion, the quantity of which predominates, the solutions contain mainly chromium, nickel, and molybdenum; also present may be copper, vanadium, tungsten, cobalt, manganese, and many others, including titanium. Titanium, for example, can be treated, say, in the form of sheet titanium, with the same solutions used on high-grade steels. For reasons of pickling technology, resulting spent solutions must be discarded when they contain only 40-50 g/l ferric ion, although they may then still contain unused nitric acid in high concentrations, e.g., 150 g/l. This requirement creates great problems involving neutralization and waste sewage disposal. Additionally, of course, operating costs, owing to poor utilization of chemicals, inter alia, are very substantial.
As will be evident from the description, infra, of the present invention, a surprisingly straightforward and simple, and equally surprisingly effective, process has been discovered. In other words, major problems in the nature of those inherent in the prior are obviated. Furthermore, the novel process which will be described hereinafter differs from this prior art in that extraneous acid, extraneous salt, extraneous solvent, or the like, are not added to spent pickling solutions; further, the solutions are not contacted with anion exchange resins, which resins are never perfectly resistant to nitric acid solutions.